Thermal processing systems are commonly used to perform a variety of semiconductor fabrication processes, including but not limited to oxidation, diffusion, annealing, and chemical vapor deposition (CVD). Most conventional thermal processing systems employ a processing chamber that is oriented either horizontally or vertically. Vertical thermal processing systems are recognized to generate fewer particles during processing, which reduces substrate contamination, are readily automated, and require less floor space because of their relatively small footprint.
Many conventional thermal processing systems include a structural outer tube defining a vacuum vessel, a cylindrical liner inside the outer tube and surrounding a processing space, a heater disposed outside of the outer tube, and a gas injector comprising an annular quartz conduit or tube. A carrier or boat, which is supported on a pedestal in vertical thermal processing systems, holds a stack of substrates in the processing space. The boat includes uniformly spaced slots that receive the individually held substrates. Confronting faces of adjacent substrates in the stack are separated by a narrow gap for process gas flow. The gas injector includes an injection section extending along a major portion of the length of the liner and a line of gas injection openings in the injection section that inject process gas into the processing space. The gas injector also includes a gas delivery section seamlessly coupled with the injection section for delivering a continuous stream of process gas to the injection section.
The inlet and delivery sections of conventional gas injectors comprise continuous lengths of a tubular conduit having an unbroken fluid lumen with a round, annular cross-sectional profile of uniform inner diameter and cross-sectional area. Typical inner and outer diameters for the tubular conduit are about 11.4 mm and about 14.0 mm, respectively.
The liner is equipped with a series of disk-shaped gas exhaust openings that also extend along the length of the liner. The gas exhaust openings are located diametrically opposite to the gas injection openings in the gas injector, which promotes a cross flow of process gas across the diameter of the liner and the substrates held by the boat in the processing space radially inside the inner diameter of the liner. The gas exhaust openings communicate with an annular pumping space defined between the liner and the outer tube. A pumping port in the outer tube communicates with the annular pumping space. The annular pumping space is evacuated by a vacuum pump through a foreline coupled with the pumping port.
The process gas streams directed through the gaps between adjacent substrates reacts with the constituent material of the substrates to form a surface layer or, alternatively, to promote a different physical or chemical surface process. The thickness and composition uniformity of the layer formed on the substrates is sensitive to various factors such as the uniformity of the gas injection, the cross flow of the process gas, and the exhaust of the reaction products and unreacted process gas. The internal resistance of the gas injector, which is proportional to the pressure drop across each gas injection opening, may be a dominant factor in determining the final flow distribution for gas injected from the gas injector into the processing space. To improve layer uniformity, conventional thermal processing systems configure the gas injection openings and gas exhaust openings to promote uniform cross flow of the process gas.
One conventional measure often taken is to dimension the gas injection openings so that their diameter is larger near the terminal end of the gas injector most remote from the substrate nearest to the process gas entry point into the liner. The disparity in the diameter may be significant between gas injection openings near the gas entry point and gas injection openings near the remote terminal end of the gas injector. Each successive gas injection opening represents a pressure drop to the process gas flowing inside the injection section of the gas injector to downstream gas injection openings. The changing diameter attempts to compensate for changes in the pressure of the process gas flow that occurs along the length of the gas injector. For example, the diameter of the gas injection openings may be about 0.5 mm over the majority of the length of the gas injector and about 0.8 mm near the remote terminal end of the gas injector. As a result, the mass flux through the larger gas injection openings may be significantly greater than the mass flux through the smaller gas injection openings, which impacts the distribution of the process gas injected into the processing space along the length of the gas injector.
Another conventional measure often taken to improve process uniformity is to progressively increase the diameter of round gas exhaust openings that are defined along the length of the liner. Specifically, a gas exhaust opening having the smallest diameter is located near the pumping port and foreline and the diameter of adjacent gas exhaust openings increases with increasing separation between the pumping port and the foreline. Consequently, the gas exhaust opening having the largest diameter is the most distant gas exhaust opening from the pumping port and foreline.
Despite reliance on these conventional measures, thermal processing systems may form layers on the substrates during a processing run that exhibit thickness and composition non-uniformities. Specifically, the layers formed on a batch or lot of processed substrates may exhibit a thickness and composition dependence that is contingent upon their location in the processing space relative to the pumping port and the gas injector. Moreover, these conventional measures may also be ineffective for preventing thickness and composition non-uniformities among different lots of processed substrates.
The use of progressive-diameter gas exhaust openings introduces non-uniformities into the dispersion of the gas during cross flow. Specifically, different points on the circumference of each of the round openings are closer to the pumping port and foreline than other points on the circumference. The resulting pressure differential across the width of each gas exhaust opening may cause different process gas flows across the different faces of adjacent substrates, which promotes layer non-uniformities. Any gas exhaust openings located either above or below the boat may be characterized a low conductance to the pumping port and, thereby, result in a significant loss of process gas near the top and bottom of the substrate stack.
Any open space between the pumping port and the adjacent base of the boat may also contribute to thickness and composition non-uniformities among substrates processed in conventional thermal processing systems. These open spaces may detrimentally influence the pumping conductance, which is a measure of the ease with which gas will flow through a section of a vacuum system. Specifically, any open space between the pedestal and boat in thermal processing systems represents a substantially empty volume that is located between the gas exhaust openings in the liner and the pumping port. The flow of the exhausted process gas in the annular pumping space encounters this open space before reaching the pumping port. As a consequence, the flow of the exhausted process gas flow may be impeded and retarded, which may reduce the pumping conductance.
There is thus a need for a thermal processing system with improved process gas delivery and method for delivering process gas in a thermal processing system that overcomes these and other deficiencies of conventional thermal processing systems.